Transistors have a dielectric layer that electrically isolates a gate electrode from a channel typically positioned in a substrate. The dielectric is desired to have low gate current leakage. A common dielectric layer material is silicon dioxide. Silicon dioxide has a dielectric constant of approximately 3.9. Future generation devices require reduction in the silicon dioxide gate dielectric thickness. Reducing the gate dielectric thickness typically increases the gate current leakage.
As silicon dioxide gate dielectric layers are made thinner, a problem with controlling the uniformity of the thickness of the layer and controlling the manufacturing reliability becomes more difficult. As a result, materials having a higher dielectric constant than silicon dioxide have been proposed as a replacement of the silicon dioxide. Others have recognized that a thin silicon dioxide based interface is required beneath a higher dielectric constant material to improve electron and hole mobility in the transistor channel. This silicon dioxide based interfacial layer between the substrate and the higher dielectric constant material can be formed by either: (1) a chemical oxide grown during a chemical cleaning of the substrate; (2) formation of a deposited or thermally grown silicon dioxide layer; or (3) formation during the deposition of the high dielectric constant material itself as a result of oxygen coming into contact with a silicon substrate.
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